1. Field of the Invention
The present invention relates to a method and apparatus for welding polymer fabrics using ultrasound. More particularly, but not exclusively, it relates to a method and apparatus for welding polymer fabrics or the like comprising a pair of ultrasonically vibrated horns acting on layers of fabrics to be welded.
2. Discussion of the Background
It is well known that certain polymers absorb ultrasound readily and by virtue of that property, offer scope for fusion bonding and other joining techniques. These phenomena have established a general application in the welding of engineering plastics, covering a wide range of techniques and encompassing products in many different markets. Of equal interest but less widely practised is the application of ultrasound to welding of woven or non-woven polymer fabrics. Considerable effort has been expended in developing ultrasound processing equipment for the manufacture of high strength non-woven or reinforced fabrics and specialised elasticated products.
One method of undertaking ultrasonic welding is disclosed by the present applicant in an earlier patent number GB 2299538 B, but other similar procedures are also known from U.S. Pat. No. 835,068, WO 94/11189, U.S. Pat. Nos. 249,416, 4,400,227, GB 8805949, U.S. Pat. No. 4,305,988, FR 2255023 and WO 95/09593.
However, not all the methods disclosed are entirely satisfactory and it is therefore an object of the present invention to provide a method and apparatus for welding polymer fabrics using ultrasound, which method is both effective, quick and is able to deal with different materials and different thicknesses of material.
Preferred polymer fabrics to be welded by the method and apparatus of this invention are those containing nylon and polyethylene, although other materials may also be welded.
According to a first aspect of the present invention, there is provided an apparatus for welding ultrasonically two or more layers of fabric comprising two opposed ultrasonically vibratable horns so spaced one from another that the layers of fabric may be passed between them.
Preferably at least one, optionally each, of the two horns is vibratable in axial mode.
Where both horns are vibratable in axial mode, the relative phase between the vibrations of the horns may be varied, whereby for constant amplitude vibrations the energy imparted may vary between a minimum at phase difference Ø=0xc2x0 and a maximum at phase difference Ø=180xc2x0.
Alternatively the frequencies at which the horns are adapted to vibrate may be so varied that one is vibratable at a frequency plus or minus between 1 and 20%, preferably in the region of 10%, of the frequency of the other.
In another embodiment, an axially vibratable horn may have an angled end face, and a second axially vibratable horn may vibrate in a direction substantially perpendicular to said end face.
The welding apparatus described above may further include a source of air directable generally toward a zone in which welding takes place.
The source of air may be a passageway extending substantially longitudinally of at least one of the welding horns to exit substantially centrally of the welding zone.
Alternatively the source of air or other cooling fluid may be a passageway longitudinal of one or both welding horns, the or each said passageway difurcating to permit exit of air through a plurality of annularly spaced outlets.
In another embodiment where one horn is vibratable in an axial mode, an opposed cooperating horn is vibratable in a torsional mode.
According to a second aspect of the present invention, there is provided a method of welding ultrasonically two or more layers of fabric comprising the steps of providing two opposed ultrasonically vibratable horns spaced one from another, and passing the layers of fabric between them whilst vibrating ultrasonically the horns.
Preferably at least one, optionally each, of the two horns is vibrated in axial mode.
Where both horns are vibrated in axial mode, the horns may be vibrated with a relative phase difference between them, said phase shift being variable between Ø=0xc2x0 for minimum energy and Ø=180xc2x0 for maximum energy.
Alternatively the horns may be so vibrated that one vibrates at a frequency plus or minus between 1 and 20%, preferably in the region of 10%, of the frequency of the other.
In another embodiment, one horn is vibrated in an axial mode whilst an opposed co-operating horn is vibrated in a torsional mode.
The method described above further includes the step of directing air or other cooling fluid toward a zone in which welding takes place.